DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
coarse red sediments deposited as levees during periods of high flow. This has produced the
texture contrast soils, which have a well-drained coarser surface soil over red clays. Severe
scalding is common on these soils that have been extensively and often heavily grazed.
Scalding results from loss of the surface soils exposing the underlying red clays.
Deep red earths have formed on coarse-grained sediments in two areas in the north of the
region and calcareous red and yellow earths have formed from the aeolian sands in the west.
In the northern floodplains study (Smith et al. 1998) in the north west of the bioregion, soils
were described as:
water deposited sediments on the alluvial plains and floodplains - mostly cracking clays,
often deep and sometimes crabholey, areas of texture contrast soils (often on rises), noncracking
clays on elevated areas, sands and sandy-earths on dunes;
water deposited sediments on playas and basins - brown and grey cracking clays (heavy in
lakes, more compact in channels and floodouts, saline in playas), often surrounded by red
country;
shallow to deep soils, often loamy and red, on rolling downs and lowlands - may be gritty
or gravelly especially on upper slopes, grade into sandy red earths in drainage lines, clays
in lakes and pans; and
water-deposited sediments including moderate to deep red earths on plains - red-brown
texture contrast soils, and grey or brown clays (non-cracking, cracking or gilgaied).
The Macquarie catchment and marshes generally have rich alluvial soils but they vary
between high quality soils from basic parent rocks and poorer soils formed from the more
acid granite materials. The soils are highly weathered and leached of the soluble nutrients and
the brown to red-brown subsoil also contains concretions (Wolfgang 1998).
2.4 HYDROLOGY
2.4.1 Surface waters: rivers, catchments and wetlands
The DRP bioregion contains the Darling River, its tributaries and their catchments. Each of
these catchments has unique flooding patterns and depositional characteristics and many
rainfall events affecting these rivers originate outside the bioregion. Map 19 and Map 20
show the rivers, major wetlands, floodplains and catchments within the Barwon-Darling
basin.
Three broad river reaches can be identified along the Barwon-Darling River system within
New South Wales (Map 19). The river between Mungindi and Walgett is largely influenced
by, and constrained within, the Cobar structural ‘lineament’. From Walgett to Bourke it is not
constrained by any structural ‘lineaments’ but is influenced by the mega scale alluvial fan
morphology of the Gwydir, Namoi and Macquarie River systems. South from Bourke the
river is again constrained by a structural ‘lineament’ - the Darling ‘lineament’ which
continues into the Murray River basin (Lloyd et al. 1994).
Rivers
The Namoi, Gwydir, MacIntyre Rivers (and associated rivers) rise in the Great Dividing
Range (North East Tablelands Bioregion) and flow across the Nandewar Bioregion.
Relatively high and reliable summer rainfalls feed these waterways. The Macquarie River
also originates in reliable rainfall regions of the Great Dividing Range and flows into the DRP
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